Thieves Guild

A thieves' guild is a guild of organized thieves and rogues. Most major cities in Faerûn, and some smaller ones, have one or more thieves' guilds. Some are powerful enough to hold significant political power, which they often obtain through bribery and intimidation. Most local guilds are short-lived groups that revolve around a particular individual.
Members of thieves' guild often serve one of the following roles, which are divided into divisions in larger guilds: assassins, beggars, bounty hunters, burglars, con artists and tricksters, cutpurses and pickpockets, enforcers and thugs, racketeers, scouts and spies, fences, smugglers, and pirates.
The guilds can be almost impossible to find, but those that know Thieves can't (as special code used by the guilds) can find dens, black markets and areas to hide from authorities.

Structure

There is very little organisation and even knowledge of the different guilds across Faerun. Most operate independantly within their own area, be that town, city or realm. Those the amass enough power and wealthy clim the ladder within the guild and can often challenge those in power. In a cut throat business members need to be friends close and their enemies closer.
Most large guilds are run by one individual, normally an individual either unquiely talented, intimidating, particular violent or well liked amonsgt the muscle of the guild. Many guilds become a vicitm of their own success and once a guild has raised enough interest and awareness through their activities, where that being extortion, theivery or murder. Either nobles flush out the theives using aggressive search tactics normally flooding the slums and other areas with city guards. Or adventurers are hired to route out the criminals.
The most successful guilds are those where the head or leader is level headed and clever. Ensuring that they sanction crime, ensuring that an area isnt saturated or a particular type of crime isnt overly carried out. This ensures that the guilds are not at the for front of peoples mind and they mostly go unnoticed.